1. Field of the Invention
The present invention relates to a Surface-acoustic wave (SAW) device for modifying and passing a signal and, more particularly, to a SAW device which includes switches for providing connections to selected SAW Interdigital Transducer (IDT) fingers of the SAW device producing a plurality of SAW characteristics from a common set of SAW IDT fingers.
2. Description of the Prior Art
Referring to FIG. 1, a typical SAW device 10 includes an input transducer 12 and an output transducer 14. Each input 12 and output 14 transducer has a plurality of SAW IDT fingers 16 formed on a substrate 18. Interconnections between selected SAW IDT fingers 16 are provided by connecting each SAW IDT finger 16 to either a first 20, second 22, third 24 or fourth 26 sum line. A radio-frequency (RF) signal voltage 28 is applied between the first 20 second 22 sum lines which provide the voltages 28 to the SAW IDT fingers 16 of the input transducer 12 creating strains in the piezoelectric substrate 18. If the strains occur with the proper periodic pattern across the length of the SAW device 10, an acoustic surface wave will be generated. The spacing between SAW IDT fingers 16, the interconnection pattern of the SAW IDT fingers 16, and the frequency content of the applied voltage difference determines the magnitude and phase of the surface acoustic wave. The acoustic wave which is launched by this input transducer 12 travels to the output transducer 14, where it is converted to an output electrical signal. If the SAW IDT fingers 16 have the proper spacing and are connected to the sum lines 20-26 with the proper interconnection pattern, a SAW device 10 having a desired signal modification in the time and frequency domains will be generated.
SAW devices 10 formed on a piezoelectric substrate 18 are typically metal film formed from photolithography and thin film processes with either etching or lift-off techniques. These techniques are typically required since the width of each SAW IDT finger 16 and the spacing between adjacent SAW fingers 16 are on the micron or submicron level. For typical prior art SAW devices 10, the SAW IDT fingers 16 and the sum lines 20-26 are formed on the substrate 18 resulting in the SAW fingers 16, the sum lines 20-26 and the interconnection pattern between SAW IDT fingers 16 being fixed on the substrate 18. Since the SAW IDT fingers 16 and sum lines 20-26 are fixed on the substrate 18, the typical prior art SAW device 10 provides only a single interconnect pattern between SAW fingers 16 and can generate a surface acoustic wave only with fixed signal modifications in the time and frequency domains. If a different surface acoustic wave is desired, a new SAW device having a new interconnect pattern must be fabricated.
One method used to provide for more than one signal modification is to fabricate a plurality of SAW transducers on a single substrate. Then, the input or output signal can be switched among several transducers each of which provides different properties. The drawback of this method is that it gives only a small amount of flexibility and requires more space on the substrate. Therefore, what is needed is a SAW device which can provide a plurality of signal modification properties from a common set of SAW IDT fingers such that the resulting SAW device properties can be changed without the need to fabricate a new SAW device.